Antistatic or static dissipative layers are of considerable interest for a variety of industries for reducing static charge build-up which can result either in a static discharge (sparking) or in the accumulation of static charge and the attraction of dirt or conveyance problems. Static charge problems are particularly of concern during the manufacture or coating of products in a roll form containing a polymeric web, such as photographic films, adhesive tapes, magnetic recording tapes, packaging films, and transparency films, and during the manufacture of fibrous products such as carpets and brushes. The charge generated during the manufacturing or coating process results primarily from the tendency of webs of high dielectric constant polymeric film base to undergo triboelectric charging during winding and unwinding operations, during conveyance through coating machines, and during finishing operations such as slitting, chopping, cutting, rolling, perforating, and spooling.
Problems associated with the generation and discharge of electrostatic charge during the manufacture and use of photographic film and paper have been recognized for many years by the photographic industry. The accumulation of charge on film or paper surfaces can cause difficulties in support conveyance as well as lead to the attraction of dust, which can produce fog, desensitization, repellency spots during emulsion coating, and other physical defects. The discharge of accumulated static charge during or after the application of the sensitized emulsion layer(s) can produce irregular fog patterns or static marks in the emulsion. The severity of the static problems has been exacerbated greatly by increases in sensitivity of new emulsions, increases in coating machine speeds, and increases in post-coating drying efficiency. Static charge can also be generated during the use of the finished photographic product. In an automatic camera, the repeated winding and unwinding of the photographic film in and out of the film cassette can result in the generation of electrostatic charge, especially in a low relative humidity environment. The accumulation of charge on the film surface results in the attraction and adhesion of dust to the film and can even produce static marking. Similarly, high-speed automated film processing equipment can generate static that produces marking. Sheet films are especially subject to static charging during use in automated high-speed film cassette loaders (e.g., x-ray films, graphic arts films, etc.).
Reduction of electrostatic charge is particularly important during manufacture and handling of electronic components since excess electrostatic charge can damage semiconductor based components. For example, during a masking step, a pressure sensitive adhesive tape may be brought in contact with electronic components. Build-up of a high electrostatic charge during unwinding of the tape can result in static discharge when the tape contacts the electronic component. In addition, electrostatic charge can result in dust attraction which can result in misalignment of masks, inadequate exposure during photoresist exposure or pinhole formation when overcoated with a dielectric layer. Conventionally, electostatic charge is removed by the action of ionized air on the tape. However, this is typically only a temporary solution. Conductive tapes or packaging films are also desired for packaging of electronic components for customer use. For example, computer memory chips are frequently shipped in conductive packaging for upgrading the memory of a personal computer since static discharge during unpacking of the electronic components can severely damage the material.
Static electricity is also a concern during abrading, finishing or sanding operations involving insulating or semi-insulating materials such as wood, plastics, minerals and ceramics. These operations may employ an abrasive layer containing abrasive grains such as aluminum oxide, silicon carbide, diamond, silicon nitride, silicon boride, or tungsten carbide. Static electricity is generated by the constant separation of the abrasive materials from the workpiece, machinery drive rolls, idler rolls, and support pad for the abrasive product. Sudden discharge of this static charge, which can be on the order of 50 to 100 kV, can cause injury to an operator or ignition or explosion of abraded dust particles. The static charge can also cause adhesion of abraded particles, making it difficult to remove by conventional exhaust systems, resulting in excess wear or poor finishing.
Electrostatic charge also builds up easily in transparent substrates used for image displays, for example, in image display parts of TV Braun tubes. The electron beam in a cathode ray tube, which forms the TV Braun tube or the display of a computer monitor, impacts a fluorescent screen which emits red, green and blue light. When the electron beam collides with the fluorescent material a static charge is generated. The static charge can result in attraction of dust to the display screen or possible deflection of the electron beam resulting in poor image quality.
Antistatic agents are also frequently added to rubber to dissipate static charge generated by a tire moving over a surface, to polyurethane used in the sole of shoes or as floor covering to dissipate static charge resulting from repeated contact and separation of surfaces.
An antistatic agent can be incorporated in rubbers, plastics, papers, etc. or dispersed in a solution containing a polymeric binder to give a coating formulation which may be applied on various supports, sheets, webs or articles. Many of the traditional antistatic agents used in the above elements employ materials which exhibit predominantly ionic conductivity. Antistatic layers containing simple inorganic salts, alkali metal salts of surfactants, alkali metal ion-stabilized colloidal metal oxide sols, ionic conductive polymers or polymeric electrolytes containing alkali metal salts and the like have been taught in prior art. The electrical conductivities of such ionic conductors are typically strongly dependent on the temperature and relative humidity of the surrounding environment. At low relative humidities and temperatures, the diffusional mobilities of the charge carrying ions are greatly reduced and the bulk conductivity is substantially decreased. Further, at high relative humidities, an unprotected antistatic backing layer containing such an ionic conducting material can absorb water, swell, and soften. Especially in the case of roll films, this can result in the adhesion and even physical transfer of portions of an antistatic layer to a surface layer on the opposite side of the support. Therefore, it is generally preferred to use electronically-conductive materials. Many of the applications indicated above also generate heat which can alter the ionic conductivity or even result in degradation or reaction of the conductive species.
Antistatic layers containing colloidal vanadium pentoxide described in U.S. Pat. Nos. 4,203,769; 5,203,884; 5,427,835; 5,439,785; 5,637,368 and others are highly effective at providing static protection, have excellent transparency and are not significantly dependent on humidity. Colloidal vanadium pentoxide is composed of highly entangled microscopic fibrils or ribbons 0.005-0.01 .mu.m wide, about 0.001 .mu.m thick, and 0.1-1 .mu.m in length. The high aspect ratio of vanadium pentoxide gel allows excellent conductivity to be achieved at very low vanadium pentoxide coverages. In order to improve the durability of the antistatic layer and adhesion to underlying or overlying layers it is generally preferred to disperse the colloidal vanadium pentoxide gel in a polymeric binder. As disclosed in the above mentioned U.S. patents several polymer binders, for example interpolymers of vinylidene chloride, have been used for aqueous-based coating compositions. However, due to the solution chemistry and oxidative potential of vanadium oxide, the selection of compatible solvents, binders and coating aids is limited. For example, for low coating coverages the vanadium pentoxide may typically be coated at 0.05 wt % or less. At such low concentrations the vanadium pentoxide is prone to instability and flocculation. Depolymerization of the vanadium pentoxide gel into ions may also occur at low concentrations or low pH values. A film-forming sulfopolyester latex or polyesterionomer binder can be combined with colloidal vanadium pentoxide in the coating composition for improved solution stability of the coating formulation and to minimize degradation during processing of the coated layer as taught in U.S. Pat. Nos. 5,360,706; 5,380,584; 5,427,835; 5,576,163; and others. Redox reactions between the vanadium pentoxide and solvents, polymeric binder or coating aids can result in degradation of polymer properties and in alteration of the electrical conductivity of the antistatic material. Reaction between vanadium oxide and various solvents, binders or coating aids is also accelerated by an increase in temperature. Consequently, the utility of vanadium oxide antistatic layers for applications such as thermal imaging elements, various display systems, and abrading, finishing or sanding operations can be severely limited based on reactivity of vanadium oxide and other components of the desired layer when subjected to elevated temperatures.
U.S. Pat. No. 5,718,995 discloses an antistatic layer containing vanadium pentoxide gel and a specified polyurethane binder having excellent adhesion to surface treated support and an overlying magnetic layer. However, it is further disclosed that the coating composition has limited solution stability.
U.S. Pat. No. 5,203,884 describes coated abrasive articles having vanadium oxide present to reduce accumulation of static electrical charge. It is further disclosed that a sulfonated polymer is preferred to aid in securing the vanadium oxide to the abrasive article.
U.S. Pat. No. 5,637,368 describes an adhesive tape having a support, an adhesive layer and a vanadium oxide layer. It is further disclosed that a sulfopolymer is used in conjunction with the vanadium oxide layer. The sulfopolymer may be mixed with vanadium oxide or provided as a layer either over or under the vanadium oxide layer.
In addition to the aqueous-based coating compositions described above it may also be advantageous to coat antistatic layers from a solvent-based formulations. U.S. Pat. No 5,709,984 describes antistatic layers comprised of a dispersion of colloidal vanadium pentoxide gel and an interpolymer of vinylidene chloride prepared from a solvent mixture of ethanol and acetone. U.S. Pat. Nos. 5,356,468 and 5,366,544 describe vanadium pentoxide gels dispersed in cellulosic binders coated from a variety of solvents. In addition to the potential for incompatibility of binders, it is well known that vanadium pentoxide can act as a catalyst or reactant for organic solvents. Potential decomposition products can adversely impact the coating quality of the antistatic layer and potentially adversely impact the sensitometric performance of photographic emulsions.
Intercalation of various species, including cations, metal-containing complexes, organic molecules and polymers, within vanadium oxide is well-known, particularly in the catalysis field and as cathode materials for batteries. However, intercalated colloidal vanadium oxide for antistatic applications has not typically been addressed.
U.S. Pat. No. 5,659,034 describes intercalation of metal coordination complexes, particularly Zn(2,2'-dipyridyl).sub.2, between layers of vanadium oxide. The resultant intercalated vanadium oxide was described as black rod-shaped crystals which are unsuitable for antistatic applications for transparent elements such as photographic imaging elements, typical electronic packaging films, or display elements.
U.S. Pat. No. 5,073,360 describes the formation of bridged/lamellar metallic oxides having intercalated spheroidal cationic species. The preferred metallic oxide is vanadium pentoxide and the spheroidal cationic species is preferably an aluminum polyoxocation, particularly [Al.sub.13 O.sub.4 (OH).sub.24 ].sub.7.sup.+. The vanadium oxide gel can be prepared for example by ion exchange or melt quenching. The intercalated material is then isolated by filtration, dried and optionally calcined to give high surface area materials which are particularly suited as molecular sieve filters, catalysts, and catalyst supports. However, no indication is given regarding the antistatic properties of the intercalated vanadium oxide.
Intercalation of a wide variety of organic or polymeric materials between vanadium oxide layers in vanadium oxide gels is well known. Intercalative polymerization of aniline resulting in polyaniline is described in Mater. Res. Soc. Symp. Proc. V. 233, pp. 183-194, 1991 and Chem. Mater. V. 8, pp. 1992-2004, 1996. A significant decrease in oxygen concentration and a color change from red to dark blue was observed when vanadium oxide gel was added to an air saturated solution of aniline in water. Conductivity of the polyaniline-vanadium oxide material increased substantially upon aging. It was proposed that conductivity in the fresh material occurred by electron transport through the vanadium oxide framework (semiconductive) but upon aging a metallic-like conductivity dominated as polyaniline chains formed.
Poly(ethylene oxide) intercalated vanadium oxide gels were reported in Chem. Mater, Vol. 3, 992-994, 1991 and Chem. Mater, Vol. 8, 525-534, 1996 to be highly light sensitive, turning dark blue within several weeks due to exposure to room light or within several hours due to exposure to UV irradiation. Non-intercalated vanadium oxide gels were not light sensitive. In addition to a color change, the conductivity increased and solubility decreased with increasing irradiation. However, the irradiated conductivity decreased with increasing polyethylene oxide intercalation. Changes in the vanadium oxide interlayer distance due to intercalation of poly(vinylpyrrolidone) (PVP), poly(propylene-glycol) (PPG), and methylcellulose are described in Adv. Mater, Vol. 5, 369-372, 1993. Interlayer distance increased linearly for (PVP).sub.x V.sub.2 O.sub.5 . nH.sub.2 O for values of x up to 3. Furthermore, a change in the chemical nature of PVP was noted and ascribed to formation of hydrogen bonding with co-intercalated water. The interlayer spacing did not vary linearly with either PPG or methylcellulose. The interlayer distance remained constant for (PPG).sub.x V.sub.2 O.sub.5 . nH.sub.2 O with x values greater than 1, and PPG remained chemically unaltered. Particularly in the case of PPG, the samples were light sensitive as indicated above.
The above references indicate a vast array of organic or polymeric species can be intercalated within vanadium oxide gel structures. However, the intercalated material is frequently light sensitive and conductivity changes during aging. Furthermore, intercalation and subsequent reaction frequently decreases solubility of the vanadium oxide gel.
The use of polyvinylpyrrolidone in antistatic formulations is also well known. For example, U.S. Pat. Nos. 4,418,141; 4,495,276; 5,368,995; 5,484,694; 5,453,350; 5,514,528 and others include polyvinylpyrrolidone amongst an extensive list of suitable binders for antistatic materials such as tin oxide or zinc antimonate. There is no specific mention or claim to enhanced properties or stability of polyvinylpyrrolidone or other water soluble vinyl-containing polymers relative to other polymeric binders for the above mentioned patents.
U.S. Pat. No. 4,489,152 describes a diffusion transfer film having an opaque layer consisting of carbon black having 2-10 percent polyvinylpyrrolidone based on the weight of carbon black. The addition of polyvinylpyrrolidone having a molecular weight of about 10,000 to the carbon black layer was found to improve the silver transfer process. However, there was no indication of antistatic properties nor of formulation stability for the carbon black layer.
U.S. Pat. No. 4,860,754 describes an electrically conductive adhesive material consisting of a low molecular weight plasticizer, a high molecular weight water soluble, crosslinkable polymer, uncrosslinked polyvinylpyrrolidone, and an electrolyte. The uncrosslinked polyvinylpyrrolidone is added as a tackifier.
U.S. Pat. No. 5,637,368 describes the use of colloidal dispersions of vanadium oxide for imparting antistatic properties to adhesive tapes. Polyvinylpyrrolidone and polyvinylpyrrolidone copolymers are included in a list of suitable adhesive compounds. The use of vanadium oxide in the adhesive layer is suggested, but all examples consist of a separate vanadium oxide layer and a separate adhesive layer. In addition polyvinylpyrrolidone was not demonstrated nor disclosed to give superior performance.
As disclosed in the above mentioned U.S. patents several polymers, for example interpolymers of vinylidene chloride, sulfopolyesters, polyesterionomers, and cellulosics have been used as binders for antistatic layers containing colloidal vanadium oxide. However, due to the solution chemistry and oxidative potential of vanadium oxide, the selection of compatible binders and formulation range is limited. For example, for low coating coverages the vanadium pentoxide may typically be coated at 0.05 weight percent or less. Such low concentrations result in coating formulations which are prone to instability and flocculation of the vanadium oxide gel. This creates serious difficulties in accumulation of flocculated vanadium oxide plugging solution delivery lines, filters and coating hoppers. Furthermore, flocculation can result in coating defects or "slugs" which can result in optical and electrical non-uniformities in the coating. The addition of surfactants to the coating solution may stabilize the vanadium oxide gel, however, the typically high levels of surfactant required are undesirable for adhesion and coatability of subsequently applied layers. The concern of stability has been addressed in many of the above patents. Furthermore, interaction between colloidal vanadium oxide and polymeric binders can result in limited dispersion shelf-life. In addition to the potential for incompatibility of binders, it is well known that vanadium pentoxide can act as a reactant or catalyst for decomposition of organic solvents. Decomposition products can adversely impact the coating quality of the antistatic layer and potentially adversely impact the sensitometric performance of photographic emulsions thereby requiring careful selection of coating solvents and binders for the antistatic layer. Furthermore, due to the potential interaction of vanadium pentoxide with solvents and binders, careful consideration must be given to formulation of overlying layers, such as barrier layers and abrasion resistant layers.
Because the requirements for an electrically-conductive layer to be useful for antistatic or electrode applications in a variety of elements or articles are extremely demanding, the art has long sought to develop improved conductive layers exhibiting a balance of the necessary chemical, physical, optical, and electrical properties. As indicated hereinabove, the prior art for providing electrically-conductive layers useful for a variety of elements is extensive and a wide variety of suitable conductive materials have been disclosed. However, there is still a critical need in the art for improved conductive layers which can be used in a wide variety of elements, which can be manufactured at a reasonable cost, which are resistant to the effects of humidity change, which are durable and abrasion-resistant, which exhibit acceptable adhesion to overlying or underlying layers, and which exhibit suitable cohesion.
It is also highly desirable to provide coating formulations which have improved solution stability, improved binder compatibility and reduced catalytic or chemical activity particularly when exposed to elevated temperatures without adversely impacting the transparency and electrical-conductivity of prior art vanadium pentoxide gel antistatic layers. The present invention is directed at providing improved coating formulations that more effectively meet the diverse needs of antistatic layers, especially those for use in silver halide-based photographic elements, thermal imaging elements, display elements, electronic packing and finishing operations but also of a wide variety of other types of elements.